docs: def handle_part(self, params):
          """
          Handle a client parting from channel(s).
          """
          for pchannel in params.split(','):
              if pchannel.strip() in self.server.channels:
                  # Send message to all clients in all channels user is in, and
                  # remove the user from the channels.
                  channel = self.server.channels.get(pchannel.strip())
                  response = ':%s PART :%s' % (self.client_ident(), pchannel)
                  if channel:
                      for client in channel.clients:
                          client.send_queue.append(response)
                  channel.clients.remove(self)
                  self.channels.pop(pchannel)
              else:
                  _vars = self.server.servername, pchannel, pchannel
                  response = ':%s 403 %s :%s' % _vars
                  self.send_queue.append(response)